1. Field of the Invention
The present invention relates to a decode control apparatus, a decode control method, a computer program and a recording medium, appropriate for reverse playing back a moving picture that is encoded in accordance with moving picture experts group (MPEG) algorithm.
2. Description of the Related Art
MPEG algorithms are available as a method for compressing efficiently moving images. In accordance with the MPEG algorithm, each frame image is successively encoded (compressed) using frame correlation on a per GOP (group of picture) basis. The GOP is composed of a plurality of frames. Each frame is encoded (compressed) into images of one of an I picture, a P picture, and a B picture in accordance with the order within the GOP.
Here, the I picture is an intra-frame image that is encoded within a given frame, and the P picture is an inter-frame forward predictive encoded image that is obtained by predicting a current image based on a preceding I picture or a P picture. The B picture is a bi-directionally encoded picture that is obtained by predicting a current image based on one of the I picture and the P picture preceding in time and one of the I picture and the P picture subsequent in time.
In the discussion that follows, N represents the number of frames contained in 1 GOP, and M represents a period within which one of the I picture and the P picture appears. If the I picture (represented by “I”), the P picture (represented by “P”), and the B picture (represented by “B”) are arranged in 1 GOP to be “I, B, B, P, B, B, P, B, B, P, B, B, P, B, B”, N is 15, and M is 3.
Since the encoding (compression) process is performed using frame correlation on a per GOP unit basis in the MPEG method, reverse playback process using one MPEG decoder having ×1 speed throughput is subject to limitation. Hereinafter, the MPEG decoder is assumed to have 1× speed throughput. The reverse playback process refers to the operation in which a non-compressed video signal, namely, in a baseband signal state, is played back in a direction in time sequence opposite from the direction at which the video is taken (in the forward direction).
In one of available reverse playback methods, a moving image encoded in accordance with the MPEG algorithm is played back using a single MPEG decoder. In this method, only the I picture is decoded and played back. The reverse playback is possible, but a user is unable to specify a playback speed. The reverse playback operation results in an intermittently moving image rather than a naturally looking smoothly moving image.
In the MPEG algorithm, each frame image is encoded using frame correlation. To decode one of a first P picture and a first B picture other than an I picture, one of a P picture and a B picture, which are referenced to encode the first P picture and the first B picture, need to be decoded beforehand. As a result, a decoding process cannot be completed in time with a single MPEG decoder.
Japanese Unexamined Patent Application Publication No. 2000-175151 disclosed an MPEG decoding technique. According to the disclosure, an I picture and/or a P picture, required to decode a B picture, is decoded in advance and stored in a frame memory. The B picture is then decoded using the I picture and/or the P picture stored in the frame memory. All pictures are decoded at ×1 speed for reverse playback.
With this technique, not only the I picture in 1 GOP but also both the P picture and the B picture are decoded. In comparison with other known techniques that decode and play back only the I picture, the reverse playback operation is smooth.